1. Field of the Invention
The present invention relates generally to a tuner used in a television receiver, and more particularly, to a tuner for use in two systems (referred to as two-system tuner below) used in a television receiver capable of receiving a television signal in which sound information is multiplexed on video information in a frequency-divisional manner using a carrier and a television signal in which sound information and video information are multiplexed in a time-divisional manner.
2. Background Art
A television signal in a conventionally known standard system is a television signal in which sound information, video information and the like are multiplexed in a frequency-divisional manner. More specifically, a sound information is multiplexed on the video information in a frequency-divisional manner using a sound carrier. A typical television standard system includes three types: NTSC (National Television System Committee) system, PAL (Phase Alternation by Line) system and SECAM (Sequential Couleur A Memoire) system.
In the case of the NTSC system, sound information B is multiplexed on video information A in a frequency-divisional manner using a sound carrier signal in order to avoid superposition of an occupied frequency band of the video information A and an occupied frequency band of the sound information B as shown in FIGS. 1 and 2.
On the other hand, a time division multiplex system for a high definition television has been developed in which sound information and video information are multiplexed in a time-divisional manner, thereby overcoming the disadvantages in the television standard system. This time division multiplex system includes the MUSE (Multiple Subnyquist Sampling Encoding) system and the MAC (Multiplexed Analogue Component) system. In the case of the MUSE system, occupied frequency bands of video information A, sound information B and control data C are superimposed, as shown in FIG. 3. In addition, in a television signal in the MUSE system, the sound information B and the control data C, and the video information A exist in a divided state on the time base in one field, as shown in FIG. 4.
As described in the foregoing, since the structure of the television signal in the standard system and the structure of the television signal in the time division multiplex system are entirely different from each other, the television signal in the standard system and the television signal in the time division multiplex system do not generally use a common transmission channel.
Recently, using satellite broadcasting of a 12 (GHZ) band in Japan, an experiment was performed in which television signals in two different systems, for example, a television signal in the NTSC system and a television signal in the MUSE system were transmitted using a common transmission channel. Therefore, in the near future, it may be that one transmission channel is going to share the television signals of different systems in regular use.
If and when one transmission channel is shared by the television signals in the two systems, it will be necessary to perform processing of, for example, changing the bandwidth of a demodulation signal in accordance with the system of a received television signal and changing signal processing after demodulation. Therefore, more specifically, a control system such as a signal processing circuit in a stage succeeding to a demodulator and an AFC (Automatic Frequency Control) circuit for automatically controlling the frequency of the signal and keeping the same constant must be switched in accordance with the system of the received television signal.
In such a case, it is considered that independent tuners will be separately employed for the respective systems. More specifically, if and when television signals in different systems using a common transmission channel are received, a television signal received by an antenna 101 is sent to a BS tuner 91 or 92 corresponding to the system of the received television signal by way of a converter for satellite broadcasting (referred to as BS converter below) 2, as shown in FIG. 5. A video signal and a sound signal outputted from the BS tuner 91 are applied to an NTSC display 60. In addition, a baseband signal outputted from the BS tuner 92 for MUSE is applied to a decoder for an MUSE signal (referred to as MUSE decoder below) 70. A video signal and a sound signal outputted from the MUSE decoder 70 are applied to an MUSE display 80.
In this case, an operator must visually determine the system of the received television signal by observing a signal waveform using an oscilloscope or the like. In addition, after visually determining the system of the television signal, the operator must select the BS tuner 91 or 92 corresponding to the system and manually switch a switching switch 90 for transmitting the television signal to the selected BS tuner. Thus, the operator must do the above described work which is complicated, every time the television signal is received.
In the above described case, the determination of the system of the received television signal is not reliable because it depends on visual determination of the operator. Furthermore, a tuner for satellite broadcasting is expensive. Therefore, if tuners corresponding to the two different systems are provided in a television receiver in order to allow the television signals in the systems to be received, the television receiver becomes very expensive, so that it is difficult to put the television in commercial use because of the cost.